This invention relates generally to airfoils for gas turbine engines, and specifically to airfoil coatings. In particular, the invention concerns an abrasive thermal protective coating suitable for use on blade and vane airfoils in the turbine or compressor section of a gas turbine engine.
Gas turbine engines are configured around a core comprising a compressor, a combustor and a turbine, which are arranged in flow series with a forward (upstream) inlet and an aft (downstream) exhaust. The compressor compresses air from the inlet, which is mixed with fuel in the combustor and ignited to produce hot combustion gas. The combustion gas drives the turbine, and is exhausted downstream. Typically, compressed air is also utilized to cool downstream engine components, particularly turbine parts exposed to hot working fluid flow.
The turbine section may be coupled to the compressor via a common shaft, or using a series of coaxially nested shaft spools, which rotate independently. Each spool includes one or more compressor and turbine stages, which are formed by alternating rows of blades and vanes. The working surfaces of the blades and vanes are formed into airfoils, which are configured to compress air from the inlet (in the compressor), or to extract energy from combustion gas (in the turbine).
In ground-based industrial gas turbines, power output is typically provided in the form of rotational energy, which is transferred to a shaft and used to drive a mechanical load such as a generator. Weight is not as great a factor in ground-based applications, and industrial gas turbines can utilize complex spooling systems for increased efficiency. Ground-based turbines are also commonly configured for combined-cycle operations, in which additional energy is extracted from the partially-cooled exhaust gas stream, for example by driving a steam turbine.
Aviation applications include turbojet, turbofan and turboshaft engines. Most modern fixed-wing aircraft employ a two or three-spool turbofan configuration, as opposed to the older turbojet design, while rotary-wing aircraft (e.g., helicopters) are typically powered by turboshaft engines. Aviation engines also power accessory functions such as pneumatics, hydraulics and environmental control, for example via a bleed air system or electrical generator.
Turbofan engines use a forward fan or ducted propeller to generate thrust via bypass flow, which is directed around the main engine core. Most turbofans have a direct drive coupling the fan to the low-pressure turbine spool, but some advanced engines utilize a reduction gearbox for independent speed control, reducing noise and increasing engine efficiency. Subsonic aircraft typically employ high-bypass turbofans, in which most of the thrust is generated from bypass flow. Low-bypass turbofans tend to be louder and somewhat less fuel efficient, but provide greater specific thrust and are generally used for high-performance aircraft.
In gas turbine engine design, there is a constant need to balance the benefits of increased pressure and combustion temperature, which tend to improve engine performance, with wear and tear on the airfoil surfaces, which tend to decrease service life. In particular, there is a need for protective coating systems that reduce the thermal and erosive effects of the working fluid flow. There is a particular need, moreover, for protective coating systems that are adaptable to a variety of different engine configurations and airfoil designs, including rotor blade and stator vane airfoils exposed to high-temperature working fluid in the compressor or turbine section of a gas turbine engine.